In recent years, as the use of the Internet has increased, electronic apparatuses that use CPUs running at high clock frequencies in a sub-microwave band (0.3 to 10 GHz), electronic apparatuses that use high frequency bus, and telecommunication apparatuses that utilize radio waves have been increasing, such as personal computers, home appliances having information processing functions, wireless LAN, bluetooth-equipped apparatuses, optical module, mobile telephones, mobile information terminals and intelligent road traffic information system. This trend leads to a society of ubiquitous computing that requires devices of higher performance with high-speed digital information processing function and low-voltage driving. However, as such apparatuses become popular, concerns have been increasing on the problems related to the electromagnetic interference such as malfunctioning of an apparatus that emits electromagnetic radiation or other apparatuses and health threats to humans. For this reason, such an apparatus is required to minimize the emission of unnecessary electromagnetic radiation so as not to affect its own operation and that of other apparatuses and so as not to cause adverse effects on the human body, and to operate without malfunctioning when subjected to electromagnetic radiation emitted by other apparatuses. Measures to prevent such electromagnetic interference include the use of an electromagnetic radiation shielding material that reflects electromagnetic radiation and the use of an electromagnetic radiation absorbing material.
As a means for preventing electromagnetic interference between electronic apparatuses, electromagnetic radiation shielding material is provided on the surface of the housing of the electronic apparatus or between the electronic apparatuses so as to block electromagnetic radiation (inter-system EMC). As a means for preventing electromagnetic interference within an electronic apparatus, electronic components and circuits are covered with electromagnetic radiation shielding material so as to prevent the electronic components and circuits from interfering with each other and resulting in malfunction, and suppressing the processing speed from decreasing and signal waveform from being distorted (intra-system EMC). Particularly in near-field environments such as within an electronic apparatus, it has been required to suppress the generation of electromagnetic noise by providing electromagnetic noise suppressing measures to electronic components that are the sources of the electromagnetic noise or to suppress the interference between signals thereby to improve the transmission characteristic (micro EMC).
Electronic apparatuses and electronic components are recently required to have higher performance and become smaller and lighter in weight, and the electromagnetic noise suppressor used in these apparatuses or components is also required to have high electromagnetic noise suppressing effects in a high-frequency band such as a sub-microwave band, be smaller and lighter in weight, and be easy to carry out by the work which takes measures with electromagnetic noise suppressing measures.
As the electromagnetic radiation shielding material, for example, an electromagnetic noise suppressor including a mixture of two kinds of soft magnetic material powder having different mean particle sizes, namely soft magnetic material powder particles having morphological magnetic anisotropy, that are dispersed in an organic binding agent is disclosed in Japanese Patent Application, First Publication No. Hei 9-35927.
In the publication described above, the electromagnetic noise suppressor is disclosed to have anisotropic magnetic fields of different intensities so as to demonstrate a plurality of magnetic resonances and different values of the imaginary part of complex magnetic permeability (μ″) that correspond to different frequencies are superposed, thus resulting in the distribution of the imaginary part of complex magnetic permeability (μ″) over a wide range of frequencies. The imaginary part of complex magnetic permeability (μ″) is a magnetic loss term required for absorbing electromagnetic radiation, and it is said that high electromagnetic noise suppressing effect can be achieved as the imaginary part of complex magnetic permeability (μ″) is distributed over a wide range of frequencies.
As another electromagnetic radiation shielding material, an electromagnetic radiation absorbing material having a composite structure of flake-shaped powder of iron nitride (Fe16N2) and a resin is disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-53487.
In the publication described above, it is described that, when the magnetic material has a high value of saturation magnetization Is, value of fr(μ′−1) that represents the limit of magnetic permeability increases, thus limitation line shifts toward higher frequency, so that higher magnetic permeability is achieved at high frequencies. As a result, it is claimed, that the use of iron nitride that has the highest saturation magnetization among various magnetic materials enables it to achieve higher magnetic permeability at higher frequencies with the resonance frequency fr reaching about 5 GHz. It is also described that the resonance frequency can be freely varied in a range from several hundreds of MHz to near 10 GHz by controlling the composition of the resin, heat treatment conditions, shape of the iron nitride particles and/or aspect ratio. An example of application is shown where ICs mounted on a wiring board are covered, together with the leads thereof, with an electromagnetic absorbing material in the state of a paste.
As another electromagnetic radiation shielding material, a NiZn ferrite thin film is known that can be used to suppress electromagnetic interference (EMI) in the sub-microwave band (Masaki ABE et al., “Application of Thin Ferrite Film and Ultra-Fine Particles formed in Aqueous Solution to Microwave/Nano-Biotechnology”, pp 721-729, No. 6, Vol. 27, 2003; Journal of The Magnetics Society of Japan).
This publication describes a NiZn ferrite thin film having the resonance frequency increased to 1.2 GHz. It is also described that the NiZn ferrite thin film is formed by plating on the surface of lead wires or semiconductor devices of a circuit by spin spraying process, and the NiZn ferrite thin film absorbs noise current before electromagnetic noise is generated from the noise current.
However, the electromagnetic interference suppressor (Japanese Unexamined Patent Application, First Publication No. Hei 9-35927) that is claimed to have imaginary part of complex magnetic permeability (μ″) distributed over a broad range of frequencies is made by simply increasing the imaginary part of complex magnetic permeability (μ″) partially, as indicated by the μ-f characteristic diagrams of FIG. 2 and FIG. 3 of this publication, and has magnetic resonance frequency lower than 2 GHz. Also the values of the imaginary part of complex magnetic permeability (μ″) shown in the μ-f characteristic diagrams are only those for frequencies up to 2 GHz, thus it is impossible to prove sufficient electromagnetic noise suppressing effect over the entire sub-microwave band.
With regards to the electromagnetic radiation absorbing material having composite structure of flake-shaped powder of iron nitride and a resin disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-53487, it is described that flake-shaped powder of iron nitride (Fe16N2) in quasi stable structure is made by thin film forming process such as vacuum vapor deposition, sputtering, CVD, MBE or the like, although details are not known since no examples are described. However, it is difficult to stabilize the crystal structure of iron nitride (Fe16N2), and iron nitride of stable structure is also included. Thus it is difficult to make flake-shaped powder of iron nitride (Fe16N2) that has sufficiently high saturation magnetization. It is also very difficult and impractical to make flake-shaped or disk-shaped fine powder of iron nitride (Fe16N2) by using a mask. It is described that the resonance frequency can be varied up to near 10 GHz by controlling the composition of the resin, heat treatment conditions, shape of the Fe16N2 particles and/or aspect ratio. However, examples are given only for those having resonance frequencies up to about 5 GHz (FIG. 5 of Japanese Unexamined Patent Application, First Publication No. 2001-53487), and there remain problems in practical application.
Although resonance frequency of the NiZn ferrite thin film proposed by ABE et al. is made higher, it is below 2 GHz and is not sufficient for an electromagnetic noise suppressor used in sub-microwave band. Also, this publication gives the values of the imaginary part of complex magnetic permeability (μ″) only for frequencies up to 3 GHz in the complex magnetic permeability spectrum (FIG. 4), while the spectrum is about to decrease at 3 GHz, indicating that the resonance frequency cannot increase further. The publication also shows the manufacture of a NiZn ferrite thin film by directly plating onto copper wires and semiconductor devices of a circuit, as an example of application. Since the plating solution contains cations of Na, etc. and anions such as chlorine and nitrous acid, it requires careful cleaning when used for semiconductor devices, resulting in increased number of operation processes.
When soft magnetic material powder or flake-shaped powder of iron nitride is used, it must be used in a large amount in order to achieve sufficient electromagnetic interference suppressing effect and electromagnetic radiation absorbing effect, the amount being usually about 90% by weight of the electromagnetic interference suppressor and electromagnetic radiation absorbing material. When soft magnetic material powder or flake-shaped powder of iron nitride is used, it is also necessary to increase the thickness of the electromagnetic interference suppressor or the electromagnetic radiation absorbing material in order to achieve sufficient electromagnetic interference suppressing effect and electromagnetic radiation absorbing effect. Thus, there has been a problem in that the electromagnetic interference suppressor or the electromagnetic radiation absorbing material has high specific gravity and is thick, and is therefore heavy.
There has also been a problem in that the electromagnetic interference suppressor or the electromagnetic radiation absorbing material is thick and makes it difficult to reduce the space requirement.
The electromagnetic interference suppressor or the electromagnetic radiation absorbing material also lacks flexibility and is brittle, since it is constituted mostly from the soft magnetic material powder or the flake-shaped powder of iron nitride, with a small content of binding agent.
With the background described above, an object of the present invention is to provide an electromagnetic noise suppressor that has sufficient electromagnetic noise suppressing effect over the entire sub-microwave band, a structure such as printed wiring board or semiconductor integrated circuit that is provided with electromagnetic noise suppressing means and a method for easily manufacturing the same.
Another object of the present invention is to provide an electromagnetic noise suppressor that requires smaller installation space and is lighter in weight, flexible, and has high strength.